This invention relates to engine fuel injection systems, and particularly to those of the type in which a fuel injector having an open condition and a closed condition controls the flow of fuel from a constant pressure source.
In such a fuel injection system, apparatus is provided to generate fuel injection pulses at a predetermined frequency or repetition rate and having durations determined according to one or more environmental and/or engine operating parameters. The apparatus is further effective to apply the fuel injection pulses to the fuel injector to activate it to its open position for the duration of the pulses and to close it between said pulses. Since the flow of fuel through the injector is essentially constant while it is in its open position, the durations of said pulses determine the fuel flow rate to the engine.
However, at the beginning and end of each of the fuel injection pulses, while the injector is opening and closing, the fuel flow through the injector is not constant; and this introduces some inaccuracy between the desired fuel flow rate as embodied in the fuel injection pulses and the resulting fuel flow rate through the injector. When the fuel injection pulses are relatively long in duration, this inaccurary is small as a fraction of the total fuel flow rate and is essentially negligible. However, at low engine speeds and loads, when the fuel flow rate is small and the fuel injection pulses thus must be comparatively short, the inaccuracy is a greater fraction of total fuel flow. It therefore may be desirable, for some engines, to define a minimum injection pulse width, below which an undesirable degree of inaccuracy is introduced into the fuel flow rate. For a particular engine, if such a minimum desired fuel injection pulse width exists and if the corresponding fuel flow is greater than a fuel flow rate expected in the normal course of operating such an engine, such as at idle, it is desirable to provide some means to eliminate this inaccuracy or at least minimize its effect on engine operation.